An oxygen evolution reaction (those with abbreviating to “OER” hereafter.) is a reaction which arises in the oxidation process of water. This reaction is an important energy conversion reaction in the charge reaction of a metal-air battery, and the direct water decomposition reaction by sunlight (refer to nonpatent documents 1, by Fabbri E. et al., and 2, by Subbaraman R. et al.). Hereinafter, the oxygen evolution reaction (OER) and “the catalyst for oxygen evolution reactions (it is also called the catalyst for OER)” as used in this description are explained by exemplifying these reactions.
For example, the direct water decomposition reaction by sunlight is represented by the following reaction formulas (I) and (II).(Cathode) 2H++2e−->H2  (I)(Anode) 2H2O->O2+4H++4e−  (II)
That is, in a cathode, hydrogen is evolved and an oxygen evolution reaction occurs in an anode. The catalyst which promotes this oxygen evolution reaction is a catalyst for oxygen evolution reactions.
When a discharge reaction of the metal-air battery is shown in the case of divalent metal (Zn) as a metal, the discharge reaction is represented by the following reaction formulas (III)-(V):(Negative electrode) Zn+2OH−->ZnO+H2O+2e−  (III)(Positive electrode) O2+2H2O+4e−->4OH−  (IV)(Overall reaction of discharge) Zn+1/2O2->ZnO  (V).
On the other hand, a charge reaction as a reverse reaction thereof is represented by the following reaction formulas (VI)-(VIII):(Negative electrode) ZnO+H2O+2e−->Zn+2OH−  (VI)(Positive electrode) 4OH−->O2+2H2O+4e−  (VII)(Overall reaction of charge) ZnO->Zn+1/2O2  (VIII).
That is, it becomes the oxygen reduction reaction (IV) during discharge, and it becomes the oxygen evolution reaction (VII) during charge. Therefore, since the oxygen evolution reaction (OER) of the present invention is represented by the above-mentioned reaction formula (VII), it turns out that OER is a reaction in the positive electrode during charge. Thus, in order to raise the charging efficiency of the metal-air battery, development of the catalyst (catalyst for oxygen evolution reactions) which raises the oxygen evolution reaction efficiency of reaction formula (VII) becomes important.
Since an oxygen evolution electrode (also called as an “air electrode” hereafter) is put to oxygen environment with high potential during charge, a catalyst and an electrode material are required to have high oxidation resistance.
From the past, as a catalyst which can meet these demands for the oxygen evolution reaction, noble metal oxide catalysts, such as RuO2 and IrO2, are used, and nano particle forms etc. are further tried for the improvement in performance (for example, refer to Non-patent Document 3, such as Lee, Y. et al.).
However, the demand to develop a catalyst which exceeds the performance of the noble metal oxide catalysts of the conventional and which is excellent in cost effectiveness for the oxygen evolution reaction is not yet attained.
One of the trials to this demand is use of a perovskite oxide catalyst. Noting that it may become the material excellent in cost effectiveness, since a perovskite oxide catalyst has comparatively high catalytic activity to OER, many examinations have been applied to the perovskite oxide catalysts (for example, refer to the non-patent document 1, such as Fabbri, E., as a review article.). Patent document 1 regarding perovskite oxide La0.7Sr0.3CoO3 as an air electrode catalyst material in a metal air battery, and similarly, refer to patent document 2 regarding perovskite oxide La1-xSrxFeO3 (provided that, x=0.1-0.2) as an oxygen generating electrode material. However, the OER reaction activity of these perovskite oxide catalysts and stability over repeated use are not enough yet as shown in FIG. 1 and FIG. 2, and a further improvement is desired.
On the other hand, Yamada et al. of the present inventor succeeded in synthesizing a perovskite oxide CaCu3Fe4O12 of an A-site ordered of a novel structure for the first time in the world by using a high pressure process (refer to non-patent document 6 of I. Yamada et al.). In the patent document 3, in order to correspond to the high-density and large-capacity of the optical component in recent years, prevention or inhibition of thermal expansion was considered as a subject. As a result, as a metal oxide material the volume of which decreases in a practical temperature range, the A-site ordered perovskite oxides which have compositions of LaCu3Fe4O12 and BiCu3Fe4O12 are disclosed in the document. However, in these references, there is no instruction or suggestion regarding the use of the A-site ordered perovskite oxide as a catalyst for oxygen evolution reactions.